1. Field of the Invention
This invention relates generally to the preparation and application of a self-emulsifiable isocyanate and coupling agent composition to a cellulosic fiber to enhance the strength and sizing properties of the fiber.
2. Background Art
There has been increasing interest in enhancing the physical properties of cellulosic fibers, and in particular paper products. Examples of these physical properties include, but are not limited to, sizing and strength properties. Attempts to enhance the physical properties of cellulosic fibers is known in the prior art. The prior art discloses the treatment of the cellulosic fiber with various chemicals and additives that can improve or enhance the overall physical properties of the fiber.
One such chemical that has demonstrated limited success for enhancing the physical properties of cellulosic fibers is the treatment of these fibers with isocyanates. U.S. Pat. No. 3,050,437 discloses that aliphatic polyisocyanates are used to impart sizing and wet strength to a paper sheet. U.S. Pat. No. 3,325,346 discloses increased strength by treating paper with polyisocyanates in combination with polyethyleneimine.
Two significant problems limit the use of polyisocyanates for the treatment of cellulosic fibers in water borne environments. First, polyisocyanates are insoluble in water. Secondly, polyisocyanates react with chemical additives that have active hydrogen moieties, such as alcohols, amines, phenols and water. When paper or board is manufactured on conventional paper machines, water is the transportation medium that carries the fiber from the pulp generation process, through screening and cleaning to improve fiber quality, and to the paper machine where the fiber is formed into a sheet. Since water comprises 97 to 99.5% of the mixture at a headbox of a paper machine prior to sheet formation, formation of insoluble polyurea derivatives is inevitable as the result of the polyisocyanate reacting with water.
The traditional paper manufacture regime utilizes the following steps: 1) water-borne fibers are carried through a number of fiber processing steps; 2) forming a mat of fiber by draining water from the fiber slurry; and 3) pressing the mat to remove additional water. In order for polyisocyanates to be useful in this process, the invention herein below has found that four conditions must be satisfied. First, the polyisocyanate must be made dispersible in water to insure good distribution in the fiber water matrix prior to sheet formation. Secondly, the polyisocyanate must be protected from water to slow down the formation of polyurea. Thirdly, the protected dispersion of polyisocyanate must be uniformly retained in the paper or board during sheet formation. Finally, the sheet must be dried to remove essentially all of the water and to complete the incorporation and/or reaction of the polyisocyanate and polyurea complex with the bleached or unbleached cellulosic fiber. If all four of these conditions are not satisfactorily addressed, the result will be paper or board with poor strength and sizing properties, deposits in forming fabrics and press felts, and excessive foaming in the sheet forming process.
Useful polyisocyanates are liquids or solids with low melting points. All useful polyisocyanates are insoluble in water and disperse very poorly in water, even when high shear mixing is applied. The large droplet size and the tendency for agglomeration of the droplets leads to poor distribution in the finished paper or board, which prevents uniform strength and sizing properties along the length and breadth of the finished product. U.S. Pat. No. 4,505,778 and Canadian Patent No. 1,087,825 disclose a method for manufacturing cellulosic papers with improved handling and strength properties by using self-emulsifiable isocyanates or an isocyanate terminated prepolymer blend. This produces some improvement in strength and sizing in the water borne process; however, with no protective layer on the dispersion and no retention enhancing mechanism, deposit formation in forming fabrics, press felts, and paper machine equipment made the application impractical. In addition, excessive amounts of foam are generated because of the production of carbon dioxide generated in the formation of polyureas. Canadian Patent No. 915,025 teaches a method for the manufacture of paper or board where polymethylene polyphenyl isocyanate is first emulsified in water by using a surfactant, and then the emulsion is fed to a stock line prior to sheet formation at 2 to 25% of fiber weight to achieve an improvement in strength and sizing. This procedure is also impractical due to deposit formation caused by the lack of a protective and retention mechanism. The presence of the external emulsifier also does not result in enhancing the sizing properties of the fiber.
U.S. Pat. No. 3,050,437 discloses that aliphatic polyisocyanates are used to impart sizing and wet strength in paper. The aliphatic polyisocyanate is dispersed in water using an anionic or nonionic emulsifier prior to the addition to the stock. This is not practical because lack of a retention mechanism can lead to deposit formation. U.S. Pat. No. 3,325,346 discloses increased strength by treating paper with the reaction product of polyethyleneimine (PEI) and an organic polyisocyanate or separate additions of PEI and polyisocyanate. The polyisocyanate is not self-emulsifying, so good distribution in the fiber is problematic, which results in foaming and deposit formation. Since polyethyleneimine has both primary and secondary amine functionality with their active hydrogens, it reacts readily with polyisocyanate. This introduces a cationic site in the newly generated addition product which enhances retention in the fiber during sheet formation. Unfortunately, the reaction with the amine groups consumes the isocyanate functionality, which greatly reduces the bonding potential with the fiber surface since the exposed amine group will only form weak ionic bonds with the fiber.
U.S. Pat. No. 4,871,798 discloses the use of a polyisocyanate reaction product with an aliphatic diol with anionic groups or groups that are capable of being converted to anionic groups. This may enhance dispersibility, but it does not increase retention because the fiber is also anionic. U.S. Pat. No. 4,904,727 discloses the use of a polyurethane compound made by reacting a polyisocyanate with tertiary or quaternary amines to enhance dispersibility or solubility and further reacting the polymer with and acrylic copolymer which is useful for sizing paper. Once again, little if any isocyanate functionality remains to bond with the fiber.
The prior art cited above does not address the three issues of dispersibility, protection of the emulsion, and retention required for successful use in the paper manufacturing process, which results in inferior products and processes.
Other prior art techniques have been developed to avoid the formation of polyurea derivatives when treating cellulosic fibers with polyisocyanate compositions. WO 93/18228 discloses a method of electrostatic spraying of water-free polyisocyanates on the surface of a preformed sheet to enhance strength and sizing. Canadian Patent No. 1,087,825 discloses a method of manufacturing a sheet of building board by first reacting a polyisocyanate with a polyol, emulsifying this reaction product in water, spraying said emulsion on wood chips or other lignocellulosic material, and hot pressing the board to cure the material. U.S. Pat. No. 4,472,550 teaches a method for the emulsification of polyisocyanates in water to act as a binder for molded articles, where the binder is sprayed on cellulosic material to produce board. WO 93/07337 discloses a method of reinforcing paper and paperboard through a surface application of a polyisocyanate to the board, containing from 3 to 15% moisture to form a polyurea coating to strengthen and to provide wet resistance to the board. The polyisocyanate is applied either undiluted or with an organic solvent. Canadian Patent No. 2,025,658 discloses a method for the production of reshapable lignocellulosic materials using polyisocyanates as a binder that is sprayed onto the mixture before heat and pressure is applied to cure the resins.
The prior art cited above illustrates the use of polyisocyanates where 100% retention of the polyisocyanate in the cellulosic substrate is obtained because it is applied at the surface of the lignocellulosic materials. Surface applications (i.e. spraying or coating) are prone to the formation of aerosols, and thus, present significant health hazard concerns. Application of the polyisocyanate in water eliminates this risk; however, as described above, polyisocyanates react with water to produce insoluble polyurea derivatives.
Thus, there is a need for a method of using polyisocyanates during papermaking that will ensure their retention and provide sizing and strength to the sheet and improve the cleanliness of the paper or board forming equipment. In addition, there is a need for a method of applying isocyanates in the absence of deposit formation.